The present invention relates generally to design and fabrication of packages for semiconductor devices. More particularly, the present invention relates to multi-layer type packages with controlled impedances and noise for signal paths and reduced inductance for power lines in order to reduce high frequency signal degradation within the package.
In the field of semiconductor device manufacture, it is necessary to provide a connection from an integrated circuit to the "outside world." This function is typically performed by a semiconductor package which houses the integrated circuit (IC) chip and includes a number of leads or pins for electrical signal and power transmission. In addition to transferring signals, power, and the like to and from the outside world, the package serves to protect the integrated circuit from environmental effects and facilitate heat dissipation.
As the operating frequencies of IC's have increased, packaging of IC's has become more critical. With increased switching speed and rate of current change, the transient voltages and consequently inductive impedance in signal paths also increases. A signal transmission path thorough a package typically includes adjacent pins or leads, and parallel vertical vias and adjacent horizontal conductive strips in the case of multi-layer packages or long parallel conductive traces formed by lead frame fingers in the case of dual in-line packages. These structures all exhibit substantial inductance, capacitance and varying impedances that result in signal degradation.
Inductance, particularly in the power and ground connections, causes waveform degradation, ground bounce and cross-talk between the various signals. Capacitive coupling between adjacent signal transmission lines also causes cross-talk which is a source of noise. Variations in signal line impedance cause signal reflections which also cause signal degradation.
The severity of these noise problems is magnified in an environment where low-voltage-swing signals, such as emitter coupled logic (ECL) signals, must operate in the presence of high-voltage-swing signals (e.g. CMOS signals), especially at higher frequencies. From the stand point of power consumption, it is desirable to employ low-voltage-swing signals whenever possible for any signal switching at high frequency. This is due to the fact that physical dimensions force the characteristic impedance of a transmission line to be no higher than about 50 ohms. To eliminate signal reflection, a 50 ohm transmission line is terminated by a 50 ohm termination resistor. The current through the transmission line is virtually constant regardless of frequency. Therefore, power in watts defined as voltage times current (P=V.times.I) or voltage squared divided by resistance (P=V.sup.2 /R), increases directly as the square of signal voltage for a given impedance.
In the case of high-voltage-swing signals, the power required to drive a transmission line with an effective capacitive loading of C is equal to charge Q(or C.times.V).times.V.times.f, or C.times.V.sup.2 .times.f. Therefore, more power is consumed to drive a higher frequency signal line. As a result, low-voltage-swing signals are much preferred at higher frequencies to maintain lower power consumption. At the same time, other signals may be required to drive circuits which require high voltage swing.
However, while low-voltage-swing signals inherently generate low noise, they are very susceptible to noise. Conversely, high-voltage-swing signals create more noise during switching, but because of their larger value are more tolerant of noise. Therefore, when the two types of signal are combined in the same environment, noise performance becomes crucial.
Prior art packages have addressed some of these problems to some extent. However, there remains a need for multi-layer packages that satisfactorily address the above problems without requiring non-standard fabrication mechanisms.
It is therefore desirable to provide a multi-layer package with reduced capacitive and inductive coupling and minimized variations in signal transmission line impedance without departing from conventional semiconductor package fabrication processes.